Stretching electronics to the limit

A material which exhibits conductivities as high as 57 S/cm and is also elastic has been created by a team from Japan [Sekitani et al., Sciencexpress (2008) doi:10.1126/science.1160309].

Researchers from the University of Tokyo, the National Institute of Advanced Industrial Science and Technology, and the National Museum of Emerging Science and Innovation used fine bundles of millimeter-long conducting dopants, single-walled nanotubes (SWNTs) to create the conducting material.

Normally, SWNTs have a tendency to aggregate together due to the strong intermolecular forces between them, making it difficult to make uniform, chemically stable materials. In this case, however, the researchers led by Takao Someya of the University of Tokyo have come up with a fabrication process that solves this problem. The SWNT bundles are produced by grinding SWNTs in ionic liquids and then uniformly dispersed in a vinylidene fluoride hexafluoropropylene copolymer matrix to produce a film with a conductivity two orders of magnitude higher than previously reported values for polymers containing SWNTs.

However, although this SWNT film is flexible it is not particularly elastic, so the team punched the material to create a net-like structure and then coated it with a dimethyl-siloxane-based rubber to increase the elasticity.

Stretching cycle tests carried out on the conducting polymer do not produce any significant change in conductivity, even after 4000 25%-stretching cycles, 500 50%-stretching cycles, and 20–50 70% stretching cycles. But, when the strain is increased beyond 110%, an irreversible change in conductance is observed.

This new conducting material has also been tried out as a component of large-scale stretchable integrated circuits (ICs) by incorporating it with organic transistors. The electrical and mechanical properties of the ICs do not deteriorate, even when stretched by up to 70%.

The next step for the team is to integrate this stretchable matrix with a two-dimensional array of pressure sensors to create flexible, artificial electronic skins.

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